Downsizing neutrals on PV systems

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NYSolar

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Can I downsize a neutral on a PV system? And where in the code can I find this? If I am connecting to a 120/208v system I have to run a neutral just for my production meter. I am told that the only reason the meter needs the neutral is for a reference point between phases and that there is no current on it. As long as it's a balanced system I should be able to downsize, correct?
 
NYSolar said:
Can I downsize a neutral on a PV system? And where in the code can I find this? If I am connecting to a 120/208v system I have to run a neutral just for my production meter. I am told that the only reason the meter needs the neutral is for a reference point between phases and that there is no current on it. As long as it's a balanced system I should be able to downsize, correct?
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If the neutral is only for voltage reference it can be downsized but must always be at least as large as the EGC. I'll look for the code reference but it is in there.
 
jaggedben said:
705.95(B)
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The only other thing I will say is that you should check with your AHJ to confirm. Austin Energy, for example, requires that the neutral be full sized from the interconnection through the PV production meter irrespective of what the NEC says.
 
NYSolar said:
Can I downsize a neutral on a PV system? And where in the code can I find this? If I am connecting to a 120/208v system I have to run a neutral just for my production meter. I am told that the only reason the meter needs the neutral is for a reference point between phases and that there is no current on it. As long as it's a balanced system I should be able to downsize, correct?
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You should also make sure that you study the inverter manufacturer's requirements in depth to confirm that you can do this. Some inverters do require it, some inverters don't require it. Some utilities and AHJs may still require some form of "effective grounding", and commonly the easiest way to do this is via the neutral.

It may be that the harmonic currents of the inverter adds up, and is carried on the neutral. I've always run a full size neutral out of ignorance for understanding what is going on. Some inverters do have the possibility for the full neutral current, such as the Fronius IG Plus 12kW WYE. If any of its individual inverting units (connected in a wye manner) fail, the imbalance will be carried back no the neutral. These inverters also have a mixed mode operation setting, which allows the inverting units to run at different power ratings to optimize efficiency, in which case any imbalance is carried by the neutral.

Most often it is the case, that if there is any serious current on the neutral for an inverter that only actively connects to the phases, the inverter will shut off with its grid sensing features.
 
jaggedben said:
705.95(B)
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I don't mean to hijack this thread, but I have a clarifying question... Could you downsize the neutral coming off of a three phase 120/208v 100Amp dedicated PV sub panel? In this case there's three single phase inverters going to that sub panel (downsizing the neutral off the inverters would be relatively useless), from the sub panel the ac output circuit goes to a production meter then to disconnect and 3P breaker in switchgear.

Sound permissible under (B)?
 
Anode said:
I don't mean to hijack this thread, but I have a clarifying question... Could you downsize the neutral coming off of a three phase 120/208v 100Amp dedicated PV sub panel? In this case there's three single phase inverters going to that sub panel (downsizing the neutral off the inverters would be relatively useless), from the sub panel the ac output circuit goes to a production meter then to disconnect and 3P breaker in switchgear.

Sound permissible under (B)?
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Debatable. I think this gets into whether AHJs treat solar combiner panels as special or treat them the same as other panels because someone could add loads.

Also, for reasons alluded to in this thread, I think what your proposing might be bad idea. It's a little over my head but I advise being careful. With 3-phase inverters it would not be the same issue.
 
Anode said:
... Could you downsize the neutral coming off of a three phase 120/208v 100Amp dedicated PV sub panel? ...
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208V or 120V inverters? I realize most likely 208V, but you didn't say. IMO, if 120V inverters, you cannot downsize the neutral.
 
jaggedben said:
Debatable. I think this gets into whether AHJs treat solar combiner panels as special or treat them the same as other panels because someone could add loads.

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Yet, technically, the neutral would be sized to calculated load (maximum unbalanced current), and you can't size for a load that's not there.
 
If I were feeding a three phase AC combiner with an array of single phase inverters, I would run a full sized neutral from the panel to the service.
 
ggunn said:
If I were feeding a three phase AC combiner with an array of single phase inverters, I would run a full sized neutral from the panel to the service.
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Followup: Unless, of course, none of the inverters touched the neutral.
 
jaggedben said:
Debatable. I think this gets into whether AHJs treat solar combiner panels as special or treat them the same as other panels because someone could add loads.

Also, for reasons alluded to in this thread, I think what your proposing might be bad idea. It's a little over my head but I advise being careful. With 3-phase inverters it would not be the same issue.
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If you mark it wit a label that says "DEDICATED PV SYSTEM PANELBOARD, DO NOT ADD LOADS", it should be able to be treated as a dedicated panelboard as far as the NEC is concerned, both today and five years from now. Most people do that anyhow, because loads will muddy the waters and put you in 705.12(D) territory with the 120% rule. I'm not sure that every electrician in the USA reads English, that's just a thought, let's put it aside.

I do wonder just how rigid the "no loads" rule is. We often put a metering assembly off this panel, that will usually not exceed 1 ampere, and still treat it as if it isn't there. Plus there could be AFCI contactor power supplies, and perhaps a convenience outlet for servicing. Nothing that will ever equal a single inverter, but a load nonetheless.
 
ggunn said:
Followup: Unless, of course, none of the inverters touched the neutral.
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Touch in what way?

A WYE configuration internally, like the 12 kW Fronii? Or a phase-to-neutral single phase connection, like most 277V single phase inverters?

Or an instrumentation and "effective grounding" way, like most inverters connect, where power is mainly connected to the phase conductors.
 
Carultch said:
If you mark it wit a label that says "DEDICATED PV SYSTEM PANELBOARD, DO NOT ADD LOADS", it should be able to be treated as a dedicated panelboard as far as the NEC is concerned, ..

...
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No cycle of the NEC contains any such provision for that label, so while this may fly as far as your AHJ is concerned, you can't say the same for the NEC. A provision like you have in mind has been proposed and rejected by the CMP in the past.
 
Carultch said:
Touch in what way?

A WYE configuration internally, like the 12 kW Fronii? Or a phase-to-neutral single phase connection, like most 277V single phase inverters?

Or an instrumentation and "effective grounding" way, like most inverters connect, where power is mainly connected to the phase conductors.
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I'd assume ggunn meant inverters that don't require a neutral connection at all. There are a few out there.
 
Carultch said:
Touch in what way?

A WYE configuration internally, like the 12 kW Fronii? Or a phase-to-neutral single phase connection, like most 277V single phase inverters?

Or an instrumentation and "effective grounding" way, like most inverters connect, where power is mainly connected to the phase conductors.
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All I meant was that it doesn't make sense to run a neutral out from an AC combiner if none of the inverters use it. It would just be a conductor to nowhere.
 
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